The use of microelectronic devices, such as integrated circuits, flat panel displays and microelectromechanical systems, has burgeoned in new business and consumer electronic equipment, such as personal computers, cellular phones, electronic calendars, personal digital assistants, and medical electronics. Such devices have also become an integral part of more established consumer products such as televisions, stereo components and automobiles.
These devices in turn contain one or more very high quality semiconductor chips containing many layers of circuit patterns. Typically nearly 350 processing steps are required to convert a bare silicon wafer surface to a semiconductor chip of sufficient complexity and quality to be used, for example, in high performance logic devices found in personal computers. The most common processing steps of semiconductor chip manufacture are wafer-cleaning steps, accounting for over 10% of the total processing steps. These cleaning steps are normally one of two types: oxidative and etch (or both). During oxidative cleaning steps, oxidative compositions are used to oxidize the silicon or polysilicon surface, typically by contacting the wafer with aqueous peroxide or ozone solution. During etch cleaning steps, etching compositions are used to remove native and deposited silicon oxide films and organic contaminants from the silicon or polysilicon surface before gate oxidation or epitaxial deposition, typically by contacting the wafer with aqueous acid. See, for example, L. A. Zazzera and J. F. Moulder, J. Electrochem. Soc., 136, No. 2, 484 (1989). The ultimate performance of the resulting semiconductor chip will depend greatly on how well each cleaning step has been conducted.
Microelectromechanical systems (MEMS) (also called micromachines or micromechanical devices) are small mechanical devices that can be made using traditional integrated circuit manufacturing techniques. Typical devices include motors, gears, accelerometers, pressure sensors, actuators, mirrors, biochips, micropumps and valves, flow sensors and implantable medical devices and systems. The manufacture of MEMS may result in a chip, or die, which contains the moving pieces of the device made from silicon or polycrystalline silicon (polysilicon) encased in silicon oxide. The die can also contain the circuitry necessary to run the device. One of the final steps in the manufacture of silicon-based MEMS is commonly referred to as “release-etch” and consists of an aqueous etch utilizing hydrofluoric acid (HF) to remove the silicon oxide to free, or “release”, the silicon or polysilicon pieces and allow them to move.
For etch cleaning steps, the composition of choice has been dilute aqueous hydrofluoric acid (HF) and, to a lesser extent, hydrochloric acid (HCl). Currently, many semiconductor fabricators employ an “HF-last” etch cleaning process consisting of an etching step using dilute aqueous HF to etch oxides.
In the wet etching of an oxidized silicon substrate, aqueous hydrogen fluoride or a mixture with an onium fluoride complex may be used as an etchant. The onium fluoride present serves to adjust the etching rate and stabilize the solution to variation in HF concentration. These buffered oxide etch solutions, or BOEs have a high surface tension and, as a result, may not adequately wet a substrate or penetrate microscopic surface features.